Low reflectivity coating and method and system for coating a substrate

ABSTRACT

A low reflectivity coating (20) is formed of a layer of carbon nanostructures (20) over a contact surface (14) of a substrate (10), from a spray incorporating the carbon nanostructures in suspension in a solvent. The carbon nanostructure layer provides a very low reflectivity coating which may be further enhanced by etching the outer surface of the coating. The layer may be etched for reduced reflectivity. Very low reflectivity coatings have been achieved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a § 371 national phase of International ApplicationNo. PCT/GB2016/052668, filed on Aug. 26, 2016, which claims the benefitof United Kingdom Patent Application No. 1515270.5, filed on Aug. 27,2015, United Kingdom Patent Application No. 1515694.6, filed on Sep. 4,2015, United Kingdom Patent Application No. 1516423.9, filed on Sep. 16,2015, and United Kingdom Patent Application No. 1602031.5, filed on Feb.4, 2016, which applications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a method of coating a substrate withcarbon nanostructures low reflectivity coating and to a method andapparatus for coating a substrate.

BACKGROUND OF THE INVENTION

For a very long time there have been efforts to produce environmentallystable coatings and devices having very low reflectivity for a varietyof industrial and scientific applications. They are important in imagingsystems, calibration targets, instrumentation, light guides, baffles,stray light suppression and many other uses.

To be commercially useful, these coatings must have a low reflectance,but as important, they should exhibit the following: be spectrally flat,low outgassing, low particulate fallout, thermal shock resistance,resistance to moisture and high resistance to shock and vibration. Thesecan be key requirements, as the coatings are often local to highsensitivity electronic detectors such as CCD or micro bolometers. Anycontamination from such coatings will inevitably collect or condense onthe detectors rendering them faulty or lowering their performance beyondan acceptable threshold.

Until recently, the best spray applied coatings have achieved areflectivity of around 2.5% in the visible spectrum (380 nm-760 nmwavelength), although some experimental studies have achieved betterresults by using CVD grown, aligned carbon nanostructures, for instancearound 0.045 to 0.5% total hemispherical reflectance (THR) whendeposited on small lab scale substrates. One example of an alignedabsorber is: US patent application: 2009/0126783 by Shawn-Yu Lin et alof Rensselaer Polytechnic Institute, entitled: “Use of vertical alignedcarbon nanotube as a super dark absorber for pv, tpv, radar and infraredabsorber application”. This document discusses a visible spectrum,highly absorbing aligned carbon nanotube film. Whilst interesting, thesealigned array absorbers are grown at high temperatures >750° C. usingcomplex and costly chemical vapour deposition (CVD) reactors and requireeven more complex catalyst steps created in Physical Vapour Deposition(PVD) reactors. This limits their use to specialist substrates withsimple planar geometries that that are capable of fitting into existingreactors, thereby limiting their commercial applications to small,simple substrates that can tolerate the high temperatures (>750° C.)used during growth of the carbon nanotubes. Also, due to the CVD methodused to grow these films, they tend to be very hydrophilic as, thepresent inventors have discovered, growth defects in the tube wallterminate to form highly polar hydroxyl, carbonyl and carboxylfunctional groups on exposure to air. This hydrophilicity rapidly causesthe film to lose its optical properties on exposure to atmospherichumidity or free water as it acts like a sponge.

A study by John H Lehman et al, “Single-Wall Carbon Nanotube Coating ona Pyroelectric Detector”, Applied Optics, 1 Feb. 2005 vol. 44, No 4, hassuggested that a low reflectivity coating formed from a solution ofcarbon nanotubes and a suitable solvent could exhibit high levels ofabsorbance when applied to pyroelectric detectors. These films createdfrom a solvent/carbon nanostructure solution have shown that they areonly able to achieve a total hemispherical reflectance (THR) of around2%, which is only on a par with the best existing commercial blackpaints. This is due to the high density of the applied film resulting inmultiple carbon nanotube sidewalls that act as an effective reflector.This allows incoming photons to be reflected without being absorbed. Thesprayed coating is also hydrophilic and so suffers from the sameatmospheric contamination issues as aligned array films. The coatingalso suffers from poor substrate adhesion.

In fields unrelated to optical absorbers, researchers have createdsolutions of solvent dispersed, functionalised carbon nanotubes forelectronic ink applications. In this type of application, it is desiredthat the ink be stable, printable and have low electrical resistanceafter printing. An example patent is US-2013/0273257, “Carbon NanotubeInk Composition and a Coating Method Thereof and a Forming Method of aThin Film Containing Carbon Nanotubes”. This document discloses thecreation of functionalised carbon nanotubes in a solvent solutioncapable of being inkjet-printed. These types of carbon nanostructureinks do not make good optical absorbers as the functionalisations andsurfactants used all contribute spectral features related to thechemical bonds in the surfactant that contribute to a large increase THRacross critical parts of the electromagnetic spectrum.

It is also known that research groups have created super hydrophobiccarbon nanotube films by depositing fluorocarbon or organosilanes on topof previously grown carbon nanotube films and powders. An example isdescribed by Kenneth K S Lau, in “Nano Letters—Super Hydrophobic CarbonNanotube Forests”. This type of coating will prevent the aligned arrayor carbon nanotube powder from taking up moisture, but the hydrophobiccoating thicknesses required to do so will reduce the film's absorbancedue to the large difference in refractive index of the continuoushydrophobic coating, that blocks of the open space between the tubes,and the fact that the carbon nanotube/filament has a reduced ability toabsorb photons when fully covered by a polymeric layer.

Prior art which discloses various methods of coating carbonnanostructures on substrates includes the following: US2016194205A;US2016053155A; US2014272199A; US2013316482A; US2013194668A;US2015298164A; US2013273257A; US2011315981A; US2011217544A;US2011111177A; US2010230344A; US2009026424A; US2008083950A;US2005013935A; and US2001004471A. However, none of these documentsdisclose any steps which result in ultra-low reflectivity coatings.

Other prior art documents include WO2003/086961 A2 (DU PONT);WO2007/075437 A2 (INTEMATIX); US2008/0171193 A1 (YI); WO2009/058763 A1(UNIDYM); US2009/0050601 A1 (PARK); and CN104558659 A (U. BEIJING).

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved method for coating asubstrate with a low reflectivity coating. The method disclosed hereinis suitable for coating a wide variety of substrates at commercialscales, including non-planar substrates and delicate substrates such asthose which become unstable at elevated temperatures. The preferredembodiments disclosed herein are able to create a highly efficientelectromagnetic (EM) absorber coating that overcomes the limitationsdiscussed above.

According to an aspect of the present invention, there is provided amethod of coating a substrate with carbon nanostructures, including thesteps of: (i) providing a suspension of carbon nanostructures in asolvent; (ii) pre-heating a substrate to a sufficient temperature tocause the solvent to evaporate when the suspension contacts thesubstrate; and then (iii) spray-coating the suspension onto thesubstrate; (iv) maintaining the substrate at a sufficient temperatureduring step (iii) in order to maintain evaporation of the solvent beingspray-coated; (v) continuing with steps (iii) and (iv) until a layer ofcarbon nanostructures has been coated on the substrate with a thicknessof at least 2 micrometres (preferably at least 3 micrometres, mostpreferably at least 6 micrometres; and (vi) plasma etching the coatingto reduce film density and create the optical cavities in the coating,adding an optical spacer to the suspension prior to the depositing stepin order to create optical cavities in the coating, or a combinationthereof.

The minimum thickness required to make a 0.2% THR reflectance at 700 nmis preferably 6 microns average coating height

Optical cavities may be created by a step of plasma etching the coating,which will reduce film density and create a coral like open structure ofnonaligned carbon nanotubes and residual amorphous carbon. This openstructure is mechanically stable and suitable for trapping andabsorbing, for example, UV-MIR (100-8000 nm) wavelengths with very highefficiency (THR of 0.2%).

Optical cavities may also be formed by the use of temporary or permanentoptical spacer particles provided in the suspension during applicationof the carbon nanostructures to the substrate, as described in detailbelow. Optical spacers are particularly suitable for trapping longerwavelengths (NIR-GHz). In some embodiments both optical spacers andplasma etching are used.

The skilled person will appreciate that the step of creating opticalcavities, whether by etching or provision of optical spacers or acombination of the two, will create additional or enhanced opticalcavities over any cavities which naturally appear in the layer of carbonnanostructures.

Where optical spacer particles are used to create optical cavities, insome embodiments there may be provided a spray including the carbonnanostructures with the optical spacers suspended in a solvent, forapplication directly to an article to be at least partially coated. Whenthe solvent dries, the carbon nanostructures with optical spacersdispersed therewithin will form a low reflectivity coating on thearticle. The material could similarly be provided in a form other than aspray, such as in a paint or ink.

Advantageously, the method also includes the step of providingprotective functionalization to the coating of carbon nanostructures,preferably without increasing the reflectance of the coating.

In preferred embodiments, the method also includes the step of providinga discontinuous protective coating over the coating of carbonnanostructures, preferably without increasing the reflectance of thecoating.

The inventors have discovered that it is possible to create a very blackcoating with a very low reflectance, by depositing carbon nanostructuresfrom a suspension onto a substrate and etching the carbon nanostructurelayer to form optical cavities in the layer, in the alternative or inaddition, optical spacers may be created by permanent or temporaryfillers to the carbon nanostructures in suspension. The carbonnanostructures are preferably carbon nanotubes (most preferablymulti-walled nanotubes; even more preferably nanotubes with ≥98%, carbonbasis, O.D.×I.D.×L 10 nm±1 nm×4.5 nm±0.5 nm×3-˜6 μm, provided by SigmaAldrich), which are deposited in a random matrix as a layer or coatingon the substrate. Previous attempts at producing low reflectancecoatings have involved forming the carbon nanotubes on a catalyst coatedsubstrate, which can involve limitations to the types of substrate whichcan be coated and limitations to the characteristics of the coating.

The method may include the step of roughening the contact surface of thesubstrate prior to depositing the layer of carbon nanostructures insuspension. Roughening may be in addition to or in place of passivation.Roughening can enhance the bonding of the carbon nanostructures to thesubstrate and improve the grazing angle performance of the film. Thesurface roughening may be by wet chemical etching, or by impacting thesurface with grit at high speed. More generally, in order to achieve afurther reduction in reflectivity, the method may include the step ofetching the substrate to be coated, which creates a surface roughness tothe exposed surface of the carbon nanostructure layer. This can beachieved by bombarding the surface of the substrate with aluminium oxidegrit of a maximum size range of 10 to 150 micrometres. The grit can besupplied either as a slurry or dry, and delivered to the surface at asuitable velocity for the hardness of the material be etched. Theetching process delivers a plurality of peaks and valleys uniformlyacross the surface. This etching step provides an improved bondingsurface, enhanced absorption and also improves grazing angle reflectanceof the coated part. The etch surface is then cleaned and dried in asuitable manner. The surface roughness also helps to create opticalcavities in the carbon nanostructure layer.

The method may include the step of providing a chemically appliedconversion coating to the, preferably mechanically or chemicallyroughened, surface of the substrate prior to depositing the layer ofcarbon nanostructures in suspension. The conversion coating applied tothe surface creates surface platelets or contact points which canenhance the bonding of the carbon nanostructures to the substrate andalso help reduce reflectance in the coating, as it produces a repeatablebut local non-uniformity in the surface structure. This helps to providea uniformly irregular surface that improves optical absorption in thefilm.

Preferably, the method includes the step of applying a bonding oradhering agent to the contact surface of the substrate prior todepositing the layer of carbon nanostructures. The bonding or adheringagent may be polyimide or other heat curable polymer that has suitablethermal stability and will not degrade when in contact with thenanostructure solution. In this embodiment, the method may include thestep of heating or curing the bonding or adhering layer during the stepof depositing the layer of carbon nanostructures thereon. This enablesthe carbon nanostructures to become partially embedded in the layer. Thesolvent used to disperse the carbon nanostructures is ‘vaporised’sufficiently quickly so as not to dilute the bonding agent or chemicallyreact with it. During curing, additional coating thickness is built upto achieve a desired overall thickness of the absorber layer.

The method may include the step of adding a permanent optical spacer orfiller, or a filler that can be later removed by heat or other means,leaving behind a cavity able to trap specific wavelength ranges of theelectromagnetic spectrum. These fillers are added to the suspensionprior to the depositing step. The optical spacer may be transmissive orabsorbing of a frequency of radiation desired to be absorbed by thecoating, or may be absorbing and designed only to create free volume inthe coating. Examples include zinc sulphide, zinc selenide, siliconcarbide, silicon nitride, styrene or amorphous carbon nanospheres. Anexample of a thermally removable spacer is naphthalene nanoparticles.When heated, they sublime leaving the original particle shape as acavity in the coating. Optical spacers of such a naphthalenenanoparticles create optical cavities within the layer of carbonnanostructures which can trap light or other radiation incident on thecoating.

In preferred embodiments, the optical spacer or filler is in the form ofparticles having an average diameter of nanometres to tens ofmicrometres depending on the wavelength of radiation to be absorbed.

In the preferred embodiments, the solvent is free of surfactants. It hasbeen found that surfactants in the coating can lead to an increase inreflectivity and a loss in blackness of the coating or, at best, asurfactant will create unwanted, large spectral features (areas ofhigher reflectivity) within the coating.

The carbon nanostructure suspension is preferably deposited on thecontact surface of the substrate by spraying. In other embodiments, thiscould be by slot-coating, dipping, spinning, brushing or electrostaticcoating.

The solvent preferably has a polarity index of from 0.2 to 6.7, morepreferably from 3.0 to 6.2 (where pentane has a reference polarity of0). In a preferred embodiment, the solvent is chloroform, although anyother suitable solvents could be used, such as ethanol, ethyl acetate,acetone, cyclohexane, tetrahydrofuran, dimethyl formamide orN-methyl-2-pyrrolidone. In scenarios in which the use of chloroform isundesirable (for example because of health risks), it is believed thatethanol or ethyl acetate should be good alternatives.

Chloroform has long been acknowledged as an exceptional solvent for arange of solutes and there is some evidence that this is due to itsunique ability to orient its molecules in polar “stacks” which may allowfor greater stabilisation of the carbon nanostructures (see Shepard etal., Chem. Commun., 2015, 51, 4770-4773). In the case of the presentinvention, it has been discovered that a highly dispersed solution ofcarbon nanostructures is undesirable as it allows for very dense packingof the carbon nanostructures on solvent evaporation. A certain degree ofagglomeration is preferred as this has been found to result in poorlypacked carbon nanostructures, with the porosity of the resultantstructure allowing for efficient light trapping. In a preferredembodiment, the solvent should have a relatively low boiling point(<150° C.) and an intermediate polarity index between 3 and 6. Thepresence of a sterically accessible nucleophile (electron donor) isbeneficial but not critical to the solvent's function.

The properties of a number of preferred solvents are listed in the Tablebelow, with water also being listed for comparison:

E-pair Boiling Polarizability H-bonding donation point/ (π*) property(α) (β) ° C. Polarity Ethanol 0.54 0.83 0.77 78.4 5.2 Ethyl acetate 0.550.00 0.45 77.1 4.4 Chloroform 0.58 0.44 0.00 61.2 4.1 THF 0.58 0.00 0.5566.0 4   Acetone 0.71 0.08 0.48 56.0 5.1 Water 1.09 1.17 0.18 100.0 9  

In preferred embodiments, once deposited, the film, composed of randomlyorientated carbon nanostructures, is reduced in density by plasmaetching for at least 900 seconds (time is specific to reactor geometryand design). The set-up of the plasma is such that it reduces thedensity of the film and opens up optical trapping cavities, so as toimprove the bulk film's absorption from UV-NIR. Typically a tenfoldimprovement in THR will be achieved by plasma etching the coating inthis way.

It should be noted that short etches of a round 15 seconds, such asthose found to improve performance on vertically aligned nanotubecoatings, will only provide a marginal or no improvement in thissolution processed coating. This is due to the difference in density ofthe starting coating and because the tubes are nonaligned. Further, ithas been shown that etching a vertically aligned nanotube coating in anoxygen plasma will provide a 30% improvement in reflectance if etchedfor 10-20 seconds, but etching for any longer period provides no furtherimprovement, and will start to damage or completely destroy the opticalproperties, and etches of 100 seconds can completely remove the coatingfrom the substrate. As the starting density of the solution processedcoating is higher, and because the nanotubes are randomly orientated inthe horizontal plane, so the plasma etching process takes longer toachieve a performance improvement.

Conventional vertically aligned CNT plasma etching gives a performanceimprovement by clustering the tips of the tubes and giving a slightreduction in density, thereby making an irregular surface and opencavities for light to be trapped and absorbed after multiple reflectionswithin the aligned CNT forest. With the solution processed film, thetubes are randomly aligned and generally horizontal to the plane of thesurface being coated. A plasma is used to etch cavities that aresuitable for trapping electromagnetic energy from UV-NIR. The etchingprocess effectively reduces the density of the bulk ‘as sprayed’ film.That reduction in density and open structure allows photons to enter and‘bounce’ multiple times until absorbed by the carbon nanotubes.

According to another aspect of the present invention, there is providedan article coated with a layer of carbon nanostructures, wherein thecarbon nanostructures are randomly disposed in the layer and free ofsurfactants, the coating including optical cavities for trappingelectromagnetic radiation.

There may be provided optical spacers in the layer of carbonnanostructures, advantageously being transmissive or absorbing of afrequency of radiation desired to be absorbed by the coating.

Preferably, the carbon nanostructures have a length of at least 2 to 10micrometres and/or are in the form of carbon nanotubes.

In practice, the carbon nanostructure layer may have a low reflectanceof 2% THR (total hemispherical reflection) in the visible spectrum, evenless in preferred embodiments.

According to a further aspect of the present invention, there isprovided a method of coating a substrate with a layer of carbonnanostructures, including the steps of: forming or obtaining asuspension of carbon nanostructures in a solvent; depositing a layer ofthe carbon nanostructures in suspension on a contact surface of asubstrate; drying the solvent, thereby leaving a coating of carbonnanostructures on the contact surface of the substrate; and creatingoptical cavities in the coating for trapping electromagnetic radiation,preferably, by adding an optical spacer or filler to the suspensionprior to the depositing step, wherein the optical spacer is or includeszinc sulphide, zinc selenide, styrene or amorphous carbon

Other features and advantages will become apparent from the descriptionand drawings which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described below, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram depicting a first step of a preferredembodiment of applying a carbon nanostructure coating to a substrate;

FIG. 2 is an enlarged schematic plan view of the coated substratefollowing the process step of FIG. 1;

FIG. 3 is a side elevational, enlarged, view of the coated substrate ofFIG. 2;

FIG. 4 is a side elevational, enlarged, view of the coated substrate ofanother embodiment of coated substrate;

FIG. 5 is a schematic diagram of the principal components of apparatusfor finishing a coating applied to a substrate according to theteachings herein;

FIG. 6 is a side elevational view of a finished coated substrate;

FIG. 7 is an enlarged schematic plan view of a coated substrateaccording to another embodiment of the invention;

FIG. 8 is a side elevational, enlarged, view of the coated substrate ofanother embodiment of the invention;

FIG. 9 is a flow chart depicting the preferred embodiments of coatingprocesses;

FIG. 10 is an SEM image of a coated sample prior to etching;

FIG. 11 is an SEM image of a coated sample after etching;

FIG. 12 is a graph showing total hemispherical reflection of the samplesof FIGS. 10 and 12;

FIG. 13 is an SEM image of a sample having a coating provided withnanosphere fillers to create optical cavities;

FIG. 14 is an SEM image of a carbon nanostructure layer deposited on asubstrate after one day of dispersion aging;

FIG. 15 is an SEM image of a carbon nanostructure layer deposited on asubstrate after seven days of dispersion aging;

FIG. 16 is a graph of reflectance plotted against wavelength of light inthe UV-visible spectrum to show the effect of using a number ofdifferent solvents;

FIG. 17 is a similar graph for the infra-red spectrum for the samesolvents as in FIG. 16;

FIG. 18 is a graph of reflectance plotted against wavelength of light toshow the effect of various different etching methods; and

FIG. 19 is an expanded graph showing the same results as that of FIG. 18but focussing on a narrower wavelength range.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Figures show in schematic form embodiments of apparatus for coatinga substrate with a carbon nanostructure layer having very low andpreferably ultra-low reflectivity. The embodiments disclosed herein haveexhibited a reflectivity considerably less than 2.5% and in most casesof less than 0.5%. The preferred embodiments have exhibited a THR ofaround 0.2% within the visible spectrum.

Referring first to FIG. 1, this shows in schematic form a method ofapplying a low reflectivity coating of carbon nanostructures onto asubstrate 10. The substrate may be any article for which it is desiredto have at least one surface with a very low reflectivity. Examplesinclude scientific instrumentation, light guides, telescope tubes,baffles, stray light suppression and many other uses.

The substrate 10 could be of any suitable material and the teachingsherein permit the material to have a wide range of characteristics,including a wide range of temperatures at which the substrate is stable,that is but not limited to melting or softening temperatures. In manypractical applications, the substrate 10 may be of a metal or metalalloy but could equally be made of a variety of other materialsincluding polymers.

FIG. 1 shows a very simple and preferred coating system for coating thesubstrate 10 with a layer of carbon nanostructures. The system includesa spray device 12, which may typically be a nozzle, array of nozzles orone or more spray slots, operable to dispense in a spray jet carbonnanostructures suspended in a solvent. The spray coats the contactsurface 14 of the substrate 10 with a layer of carbon nanostructures andwhen the solvent evaporates, typically either naturally in air at roomtemperature or by heating, there is left a dried layer of carbonnanostructures on the substrate. In some embodiments the layer isconstituted solely by carbon nanostructures, while in other embodimentsthere may be optical spacers. Further details are described below.

The carbon nanostructures are preferably in the form of carbonnanotubes, advantageously having a length of around 2 to around 10micrometres. Carbon nanotubes having such characteristics are readilyavailable for purchase and can also be manufactured by any of themethods described in the above-mentioned patent applications and also byany other known methods.

Spray coating, in the manner depicted in FIG. 1, is the preferred methodof applying a coating to a substrate as this can be readily controlledto form a uniform layer of carbon nanostructures in random orientationson a surface. Other coating methods may be used including, for example,slot-coating, dipping, spinning, brushing and the like. The carbonnanotubes may also be applied to the substrate by an electrostaticcoating method.

It is to be appreciated that where a coating process occurs over aperiod of time and/or is applied to a warm substrate, the coating maydry at the time it is applied and not in a subsequent drying step. Forexample, in the case of spraying onto a warm substrate, the solventevaporates on contact with the warm substrate, such that drying occurscontinuously as the coating is built up. Such continuous and rapiddrying can produce a more open structure of carbon nanotubes compared toa process which applies a wet coating which is subsequently dried in aseparate stage. Of course, in some embodiments the carbon nanostructuresin suspension can be applied to the substrate and then the solvent driedin a separate step after coating.

With reference now to FIGS. 2 and 3, these show in schematic form thearrangement of the carbon nanostructures 20 on the contact surface 14 ofthe substrate 10 after coating and drying. In the example of FIG. 3, thecarbon nanostructures 20 are deposited directly on the substrate 10, forwhich it is advantageous if the contact surface is roughened andconversion coated prior to the depositing step. The conversion coating,typically by a wet chemical treatment process such as Alochrom (Used onAluminium substrates) on contact surface 14. This creates platelets atthe contact surface 14 which enhance the bonding properties of thesurface. Passivation is known in the art of material surface treatment.

In some embodiments, instead of or in addition to passivation, thecontact surface 14 may be roughened prior to depositing the layer ofcarbon nanostructures. Roughening creates attachment or key points whichenhance the bonding of the carbon nanostructures to the substrate andalso help improve grazing angle reflectance. An embodiment involvesbombarding the surface of the substrate with a grit, for instance ofaluminium oxide, of a maximum size range of 10 to 150 micrometres. Thegrit can be supplied either as a slurry or dry, and delivered to thesurface at a suitable velocity for the hardness of the material to beetched. The etch surface is then cleaned and dried in a suitable manner.The etching process delivers a plurality of peaks and valleys uniformlyacross the surface. This etching process provides an improved bondingsurface, enhanced absorption and also improves grazing angle reflectanceof the coated part. The surface roughness also helps to create opticalcavities in the carbon nanostructure layer.

As explained above, once the carbon nanostructures in solution aredeposited onto the contact surface 14 of the substrate 10, the solventis evaporated, so as to leave the dried carbon nanostructures 20 as alayer on the substrate 10. In the preferred embodiment, chloroform isused as the suspension medium and this can be dried either at roomtemperature or at higher temperatures in order to remove the solventfrom the carbon nanostructure layer. In practical embodiments, anysuitable solvent may be used, preferably of a type which evaporates attemperatures between about 60° C. to about 300° C. Such temperatures aresuitable for a wide variety of substrate types.

The carbon nanostructures typically lie substantially planar to thecontact surface 14 and can be described as lying horizontally thereto.

With reference now to FIG. 4, this shows in schematic form a sideelevational view of another embodiment of article, in which there isprovided a bonding or adhesive layer 24 on the contact surface 14,between the substrate 10 and the layer of carbon nanostructures 20. Inthe preferred embodiment, the bonding layer 24 is made of a lowoutgassing polymer. A suitable polymer is polyimide, although otherpolymers may be used such as polyamides and epoxies. The layer of lowoutgassing polymer may typically have a thickness of 50-60 nanometresbut may also be thicker, for example of one or more micrometres.

Advantageously, the polymer is heated or cured during the stage ofapplying the carbon nanostructures to the substrate 10, which enablesthe carbon nanostructures to become partially embedded within thebonding layer 24. A bonding layer 24 is useful for coating substratesformed of materials which otherwise may not bond strongly directly withthe carbon nanostructures 20. In practice, the stage of depositingcarbon nanostructures on the substrate 10 continues after the bondinglayer 24 has been cured, or cooled, so as to build up a thickness ofpure carbon nanostructures 20. In this manner, the material of thebonding layer 24 does not interfere with the reflectivitycharacteristics of the carbon nanostructure layer 20. In cases where theboding layer 24 is melted or softened, the process temperature could bereduced during continued coating with carbon nanostructures, in order toharden the bonding layer 24 and allow the formation of a coating of purecarbon nanostructures above the bonding layer 24.

The skilled person will appreciate that a bonding layer may not benecessary and that in many embodiments the carbon nanostructures willattach to the substrate through Van der Waals forces.

In practice, the carbon microstructure suspension can be formed bymixing carbon nanostructures in a volume of solvent, preferably bysonication, that is by sonic induced mixing. It has been found that asonic mixing stage of 3 to 4 hours can achieve good dispersion of thecarbon nanostructures within the solvent. Suitable solvents includechloroform, cyclohexane, tetrahydrofuran, dimethyl formamide andN-methyl-2-pyrrolidone. In some embodiments, water may be used as asolvent, preferably with one or more surfactants such as SDBS (sodiumdodecylbenzenesulphonate) or Solsperse 44000 (available from AveciaInc.). The coatings depicted in FIGS. 2 to 4 provide a very black, lowreflectivity, coating on a substrate 10 and it has been found that thiscan have a substantially lower reflectivity than known paints and otherblack coatings. In this form, the coating may be more than adequate fora wide variety of applications.

The reflectivity of the coating 20 can be further reduced by processingof the structures shown in FIGS. 2 to 4 to create optical cavities inthe outer surface of the coating 20. This can be done by plasma enhancedchemical etching using apparatus of the type depicted in FIG. 5, forexample. The step of etching or roughening the outer surface of thecarbon nanostructure coating also increases the lam bertian nature ofthe surface, capturing and reflecting light over a wide range of angles,resulting in a matt appearance from every angle.

Referring now to FIG. 5, this shows in schematic form the basiccomponents of a plasma enhanced chemical etching system 50 for etchingthe outer surface of a layer 20 of carbon nanostructures on a substrate10.

The apparatus 50 includes a sealable chamber 52, in which are disposedfirst and second electrodes 54, 56. The electrodes 54, 56 are, in thisembodiment, plate-like structures which are substantially square orrectangular in plain view and shaped and sized to be able to accommodatethe shape and size of an article to be etched by the apparatus. Theelectrodes 54, 56, though, do not need to have the specific form shownin FIG. 5.

The first electrode 54 is, in the configuration shown in FIG. 5, thecathode and is coupled by a conductor 60 to an alternating currentsupply 68. Most forms of plasma have shown to provide the improvedreflectance. Optimum results have been achieved by an RF plasma, but DC,Pulsed DC and microwave frequencies can all be used. An atmosphericmicrowave, RF or DC plasma system can produce similar results.

In this particular embodiment, the electrode 54 has an array ofperforations or apertures in its side 64 which faces article 10 to befinished. Coupled to the electrode 54 is a source 76 of oxygen, carbontetrafluoride and/or other suitable material. The supply of gas from thesource 76 can be controlled by a suitable device such as a mass flowcontroller (not shown in FIG. 5).

The source of gas 76 is fluidically coupled to the electrode 54 suchthat during operation of the apparatus 50 gas may exit through thenozzle or nozzles in the face plate 64 of the electrode 54, creating aplasma for use in etching the carbon nanostructure layer 20.

The second electrode 56 is coupled to ground 62. The electrode assembly56 may be a complex structure of the type disclosed in theabove-mentioned patent publications, including provisions formaintaining a low substrate temperature in order to enable processing ofeven delicate structures.

Coupled to the chamber 52 is an outlet 58 connected to a vacuum pump(not shown in FIG. 5) able to evacuate air within the chamber 52, as iswell known in the art.

A coated structure, for example as shown in FIGS. 2 to 4, is positionedbetween the electrodes 54, 56 in the chamber 52 and then an etchingplasma is created, for instance of oxygen but preferably of a fluorinecontaining gas such as carbon tetrafluoride. The plasma is maintainedfor a sufficient period to etch the top surface of the carbonnanostructure layer 20. In some practical examples, etching will becarried out for a few minutes, up to around 15 minutes or more. Thelength of the etching stage is dependent upon the density of the carbonnanostructures forming the layer 20, the depth of the layer 20 and thedesired depth of roughness and reactor characteristics.

With reference to FIG. 6, this shows in schematic form the effect of theetching process carried out by the apparatus shown in FIG. 5. The topsurface 24 of the coating layer 20 is pitted by this process, so tocreate cavities 26 within the coating 20. The etching process willreduce the density and depth of the coating layer 20. The cavities 26act as optical cavities able to trap light and other radiation impingingon the layer 20, thereby substantially reducing the reflectance of thesurface. In an example, the reflectance of the coating can be reducedfrom around 2.0% to around 0.5% and even to around 0.2% by surfaceetching.

More specifically, in preferred embodiments, the deposited film,composed of randomly orientated carbon nanostructures, is reduced indensity by plasma etching for a period which could be at least 900seconds (time is specific to reactor geometry and design). The set-up ofthe plasma is such that it reduces the density of the film and opens upoptical trapping cavities, so as to improve the bulk film absorptionfrom UV-NIR. Typically a tenfold improvement can be achieved by plasmaetching the coating.

The embodiments described above create a coating 20 of pure carbonnanostructures. It is envisaged that the coating 20 could in someembodiments have additional constituents apart from the carrier solventused during the coating process. With reference to FIGS. 7 and 8, theseshow in schematic form two other embodiments according to the teachingsherein. In one of these embodiments, there is added to the suspensionoptical spacer elements, in the form of particles which are alsodispersed within the solvent. The optical spacers may be a transmissiveor absorbing of one or more radiation frequencies or ranges of radiationfrequencies, in dependence upon the desired characteristics of thecoating 20. In one example, the optical spacer elements may be orinclude zinc sulphide and/or selenide, styrene or amorphous carbonnanospheres, for example. The particles may have an average diameter ofa few nanometres to tens of micrometres (preferably 500 nm), independence upon the desired characteristics of the coating. In practicalembodiments, the particles may have an average diameter of around 0.1 to6 times the optical or other frequencies which it is desired that thecoating absorbs.

As can be seen in FIGS. 7 and 8, once the suspension has been depositedonto the substrate 10 and the solvent evaporated, the coating layerincludes a structure or mesh of carbon nanostructures 20 havinginterposed therewithin the optical spacer particles 30. The opticalspacer particles 30 create optical cavities within the structure whichassist in trapping light or other radiation impingent on the surface 20.

In another embodiment, instead of permanent optical spacer elements aremovable filler is used, formed of removable optical filler particles.Naphthalene is a suitable material for this purpose and the skilledperson will be able to identify others. The temporary filler is removedfrom the layer after drying, that is after removal of the solvent. Thetemporary filler particles preferably have similar characteristics interms of size and density to the permanent optical spacer elementsdescribed above. The particles can be removed, for example, by heatingor chemical removal, so as to leave optical gaps or cavities 30 withinthe coating layer 20.

In embodiments which include optical spacers or cavities within thelayer 20, it may not be necessary to carry out the etching processdescribed above with reference to FIGS. 5 and 6, although it is notexcluded that such a step could also be carried out for theseembodiments.

With reference now to FIG. 9, this shows a flow chart 100 depicting thevarious stages in the formation of an article according to the teachingsherein. At step 102, carbon nanostructures, in the preferred embodimentcarbon nanotubes, are obtained, as described above either from readilyavailable suppliers or by manufacture thereof. At step 104 the carbonnanostructures are added to a solvent, chloroform for example. At step106 optical fillers or spacers are optionally added to the solvent, inthe case where these are desired. At step 108, the carbon nanostructuresand optional fillers or spacers are dispersed in the solvent, typicallyby sonic dispersion for a period of 3 to 4 hours. It is preferred thatthe carbon nanotubes have a concentration of 1 mg/ml milligrams permillilitre of solvent. In some embodiments, after sonication thesolution is allowed to age, that is left idle for a time, which causesthe suspended carbon nanostructures to form small agglomerates (step109). This agglomeration creates greater surface roughness when thesolution is sprayed on the substrate, and so can produce a blacker, moreabsorbing film. FIGS. 14 and 15, described below, show examples ofaging.

At step 110 the substrate 10 to be coated with the carbon nanostructurelayer is prepared. This may be by roughening, at step 112 or 114, or bycoating with a low outgassing polymer, for example, at step 116. Thecontact surface may be left in its roughened state from step 112 priorto passing to the coating step 120. In other embodiments, after surfaceroughening (step 114), the substrate 10 may be passivated, at step 118,by etching or anodization. In other embodiments, the substrate may bepassivated without roughening.

At step 120, the carbon nanotube suspension is applied to the contactsurface 14 of the substrate, preferably by spraying, although this couldalso be by any other suitable method such as slot-coating, dipping,brushing and electrostatic coating. At step 122, the coating is dried soas to remove the solvent, at conditions which are appropriate for thesolvent which is used. Typically, this may be at room temperature or atelevated temperatures preferably between, for example, about 60° C. toabout 300° C.

Where a removable filler is added to the suspension, this may be removedat step 124. Similarly, where desired, the dry coating may then beetched to create optical cavities, at step 126. At step 126 there may beprovided a hydrophobic coating over the layer of carbon nanostructures.This could be done at the same time as etching, for example by use of afluorine containing plasma, in the preferred example carbontetrafluoride as a precursor, in some embodiments mixed with ahydrocarbon, preferably acetylene. In another embodiment, coating with ahydrophobic material can be carried out after etching of the topsurface. The hydrophobic coating may coat through the entire depth ofthe layer of carbon nanostructures but in preferred embodiments coatsonly partially through the layer of carbon nanostructures. Thehydrophobic coating may partially coat the tops of the surface tubes andfunctionalise tube defects. Such a coating can prevent capillary action,thereby providing waterproofing, without any increase in THR.

The process concludes at step 130 with a coated substrate.

FIGS. 10 to 15 show characteristics of example embodiments manufacturedaccording to the teachings herein. With reference first to FIGS. 10 and11, these show, respectively, SEM images of a sample of a carbonnanostructure layer deposited onto a substrate prior to and afteretching. The carbon nanotubes form a matrix of randomly orientedfilaments, or tubes, on the surface of the substrate, which are allroughly aligned with the plane of the surface of the substrate. Etchingof the carbon nanostructure layer creates cavities in the layer, as wellas reducing its density. The effect is depicted in the graphs of FIG.12, where the curve 150 shows the total hemispherical reflectance of thecoating prior to etching (that is the coating as shown in FIG. 10),while the curve 160 shows the total hemispherical reflection of thecoating after etching (that is the etched coating as shown in FIG. 11).AS can be seen, etching can provide a significant reduction in totalhemispherical reflectance and which is consistent over a greater rangeof wavelengths.

FIG. 13 is an SEM image of an example of a coating provided withpermanent nanosphere fillers dispersed throughout the carbonnanostructure matrix, creating optical cavities. Fillers of this naturehave been found to be particularly effective for absorbing wavelengthsfrom NIR to FIR (15 micrometers to 1 mm wavelength).

FIGS. 14 and 15 show the effect of allowing the dispersion of carbonnanostructures to age, when dispersed in a solution without surfactants.Over time, the dispersed carbon nanostructures in the solution willbegin to agglomerate. The inventors have found that this can bebeneficial to the formation of a low reflectance coating. FIG. 14 is animage of a carbon nanostructure coating having been aged for one day,that is left for a day after being dispersed in solution. As can beseen, the surface has a roughness, caused by agglomerated, or clumped,carbon nanostructures. FIG. 15 shows a similar coating but applied after7 days of aging. The layer is significantly rougher than that of FIG.14. Optimal aging has been found to range from one to around 12 days,after which no further advantage in terms of layer roughness isexhibited. The actual optimal aging time is primarily dependent upon thenature of the carbon nanostructures, the solution, and concentration ofthe carbon nanostructures. The skilled person will readily be able todetermine an optimal time for a specific dispersion from the teachingsherein.

The coating may be chosen to exhibit very low reflectivity over a widerange of wavelengths, while in other embodiments it can be tuned to havevery low reflectivity at specific wavelengths or wavelength ranges. Thisis possible by selective etching of the outer surface of the coatinglayer 20 and/or by selection of particles of the optical spacer or ofthe removable filler. The skilled person will appreciate also thatoptical spacers or fillers having a variety of different characteristics(for example particle sizes) can provide differing reflectivitycharacteristics to the layer 20. For instance, the layer 20 couldincorporate a plurality of different optical spacer particles or fillerparticles.

It is believed that the use of a carbon tetrafluoride and acetylene mixin the plasma creates a carbon fluorine polymer-like coating which isformed on the layer of carbon nanostructures, which is highlyhydrophobic and very stable. As explained above, it has been found thatcarbon tetrafluoride per se may be used successfully, that is without anacetylene or other reactant in the plasma, to form a hydrophobic coatingor functionalisation on the layer of carbon nanostructures. Otherprecursors can likewise be used without an additional reactant in theplasma, including for example: chlorotrifluoromethane (CF3CL),bromotrifluoromethane (CF3Br), trifluoroiodomethane (CF3I),tetrafluoroethylene (C2F4). It is also possible to use nitrogentrifluoride (NF3) and boron trifuloride (BF3), as well as pure fluorine(F2). It is believed that a fluorine or fluorine based precursor in theabsence of carbon in the precursor or a separate carbon source will forma functionalised fluorocarbon coating on the carbon nanostructure layer.

Other fluoropolymers may be used, such as polyvinyl fluoride (PVF),polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE),fluorinated ethylene-propylene copolymers (FEP), perfluoro alkoxylatedpolyfluoroolefins (PFA), and related fluorocarbon and chlorofluorocarbonpolymers, which are all good examples of thermally stable, chemicallyinert materials which can be used to increase the water resistance ofmaterials or structures.

Other embodiments use an organosilane as a precursor.

FIGS. 16 and 17 show the effect of using different solvents on thereflectivity of the resulting carbon nanostructure in accordance withthe invention. A number of bead-blasted coupons were each sprayed with40 ml of carbon nanotubes in a variety of different solvents. Thespraying was carried out under the same conditions (1 mg of CNT per mlof solvent) using a new spray gun each time. The coupons were thenhydrophobically coated using the following procedure:

Hydrophobic Processing

The absorber coating may be made hydrophobic by re-functionalization ofthe CNTs after the O₂ etching. This is achieved by exposing thedefective CNTs to a CF₄ plasma functionalization (or by means of anyprecursor able to generate F radicals in a plasma).

The displacement of carbonyl and carboxyl groups through fluorination isthought to render the structure hydrophobic due to the inability of C—Fbonds to accept protons necessary for the formation of hydrogen bonds.The fluorine content of carbon nanotubes increases with temperature upto a maximum coverage of C₂F between 250° C. and 300° C. Fluorinebonding shows two different bonding types, depending on the location ofthe F in the ring. Fluorination of the ring at low temperature is onlypossible at neighbouring carbon atoms (1, 2) generating a meta-stablestate where re-combination of the fluorine atoms and degradation of thering structure may occur through interaction with water or othersolvents. A significantly more stable configuration requires therearrangement of the F atoms to a (1, 4) configuration at opposite sidesof the ring. The strength of the C—F bond in this case increases to amore covalent character with greater stability. To reach this position,fluorine needs to migrate through the energetically unfavourable nextnearest neighbour configuration at (1, 3). At temperatures between 250and 300° C. the required activation energy to overcome thisthermodynamic obstacle is achieved. Above these temperatures, thegraphitic integrity of the tubes begins to break down into amorphousmaterial where coverage can approach CF.

Without wishing to be constrained by theory, it is thought that if afluorocarbon like CF₄ is used on its own the substrate temperature mustbe heated to at least 100° C., but preferably 250° C.-300° C. (above 300C the bond is damaged so isn't stable), and the functionalisation takesabout 5 minutes at very low power density (about 0.09 Watts/cm sq). Asolid film is not thought to be created as there is no hydrogen presentto polymerise the reactants. The fluorocarbon radicals simply displacethe carboxyl and other polar groups on the CNT defect sites whilstreducing the reflectance of the film by clustering the CNT tips. Thetime range is from 3 minutes to 12 minutes. Beyond this time the film isdamaged in terms of reflectance.

Actual process conditions in this case were as follows (these can bevaried depending on reactor design and configuration):

Pressure—0.2-1.2 torr (up to atmospheric pressure) Power density at13.56 MHz—preferred 0.09 watts/cm sq but can be from 0.01-4 watts/cm sq

Temperature—20° C.-300° C.

Duration—typically 5 minutes

Gases used—CF₄ (most F containing gases can be used)

Before hydrophobic coating, the coupons were plasma etched for 920 s.The results for the total are shown in the Table below and in FIGS. 16and 17:

After Etching 920 s THR @ 0.7 mm THR @ 5 mm Ethyl acetate 0.20% 0.23%EtOH 0.25% 0.39% Acetone 0.23% 0.46% THF 0.22% 0.67% Chloroform 0.26%0.80%

It can be seen that ethyl acetate or ethanol should be a goodreplacement for chloroform based on THR results in situations where thehealth risk posed by chloroform should be avoided.

EXAMPLES Comparative Example 1: Pre-Etched Sample

A solution of multiwall carbon nanotubes (98% carbon basis, O.D.×I.D.×L10 nm±1 nm×4.5 nm±0.5 nm×3-˜6 μm, provided by Sigma Aldrich) in ethylacetate [Tech grade, provided by ReAgent] of concentration 1 mg/ml wasprepared. The boiling point of ethyl acetate is 77.1° C.

The solution was then spray-coated onto an Aluminium 6061 gradesubstrate according to the following method:

the substrate was pre-heated to a temperature of 100° C.

(ii) the CNT solution was spray-coated onto the substrate using an IwataEclipse G6 spray gun, whilst maintaining the temperature of thesubstrate at/between 80° C. and 100° C. As a result, the solventevaporated soon after contacting the substrate surface.

(iii) the spray-coating step was continued until a layer of CNTs of atleast 2 μm was obtained. The mass of coating per cm² was measured usinga Mettler Toledo balance, model number MS204, and was found to be 1.19mgcm−2.

Total hemispherical reflectance of the coating was measured using aShimadzu UV2600 UV-Vis spectrometer with a barium sulphate integratingsphere and a Bruker Vertex 70 IR spectrometer fitted with a Pike MidIR-IntegratIR with a diffuse gold surface at a spectral range up towavelengths of 16000 nm. The results are shown in FIG. 18 (for the fullspectral range measured) and FIG. 19 (for the spectral range up towavelengths of 1400 nm).

Example 2: Etched Sample

An aluminium 6061 grade coupon was sprayed according to the procedureset out in Comparative Example 1 until the mass of the coating on thesubstrate was 1.13 mgcm⁻². The coating was then subject to an oxygenetch in a plasma reaction chamber (NanoGrowth 400 Etch) to generate therequired optical cavities. The etching process comprised a 7 minuteoxygen etch at a pressure of 0.3 Torr and a power density of 0.12 Wcm⁻².Following this the resultant sample was hydrophobically coated, in thesame plasma chamber, for 5 minutes in a carbon tetrafluoride plasma at apressure of 0.3 Torr and 250° C. substrate temperature with a powerdensity of 0.12 Wcm⁻².

The THR of the resulting coating was measured as for Comparative Example1 above and the results are shown in FIGS. 18 and 19. It can be seenthat the THR is significantly reduced compared to Example 1.

Example 3: Over-Etched Sample

An aluminium 6061 grade coupon as above was sprayed according to theprocedure set out in Comparative Example 1 until the mass of the coatingon the substrate was 0.33 mgcm⁻². The sample was then oxygen etched andhydrophobically coated as for Example 2.

The THR of the resulting coating was measured as for Comparative Example1 above and the results are shown in FIGS. 18 and 19.

The result is a significant over-etch and as a result the reflectancevalues are increased (but are still lower than those of Example 1).

Example 4: Under-Etched Sample

An aluminium 6061 grade coupon was sprayed according to the procedureset out in Comparative Example 1 until the mass of the coating on thesubstrate was 1.07 mgcm⁻². The coating was then etched in an oxygenplasma as before for 7 minutes at a pressure of 0.3 Torr but a reducedpower density of 0.045 Wcm⁻².

The sample was then oxygen etched and hydrophobically coated as forExample 2.

The THR of the resulting coating was measured as for Comparative Example1 above and the results are shown in FIGS. 18 and 19.

The result is that fewer optical cavities are created and thus thereflectance values do not reach optimum levels (but are still lower thanthose of Example 1).

Example 5: Sample Prior to Hydrophobic Coating

An aluminium 6061 grade coupon as above was sprayed according to theprocedure set out in Comparative Example 1 until the mass of the coatingon the substrate was 1.05 mgcm⁻². The sample was then oxygen etched asfor Example 2 but removed from the chamber prior to hydrophobic coating.

The THR of the resulting coating was measured as for Comparative Example1 above and the results are shown in FIGS. 18 and 19. It can be seenthat the THR is lower than the figures for Examples 3 and 4 (over themajority of the spectral range) but not as low as the THR for Example 2,which has the additional hydrophobic coating.

Polymer Underlayer

It has been discovered that a substrate having a very low reflectivitycan be produced by coating a substrate with a polymer solution,incorporating carbon nanostructures into the polymer solution while itis still wet, drying the substrate and then selectively removing thepolymer molecules to leave a nanostructure-rich layer at the surface ofthe substrate.

Accordingly, in a further aspect of the invention, there is provided amethod of coating a substrate with carbon nanostructures, including thesteps of:

(i) coating a substrate with a polymer solution which is a polymerdissolved in a first solvent;

(ii) providing a suspension of carbon nanostructures in a secondsolvent;

(iii) spray-coating the suspension onto the substrate so that the carbonnanostructures are incorporated into the polymer solution;

(iv) drying the substrate to evaporate the first and second solvents toleave a coating of polymer and carbon nanostructures;

(v) plasma etching the coating to remove at least some of said polymer.

wherein following step (iv) and before step (v) the polymers are curedin the coating

The product of this method can then optionally be used as a substrate inthe method of the first aspect of the invention defined above.

Without wishing to be constrained by theory, it is thought that theselective removal of the polymer leaves pendant nanostructure moleculesat the surface of the substrate to which a further coating can beapplied in order to result in excellent ‘adhesion’.

Exemplary (Non-Limiting) Process:

1) Synthesis of a polymer precursor. A poly(amic) acid derived fromPyromellitic dianhydride (PMDA) and 4,4-oxydianiline (ODA) in dissolvedin NMP in a 1:1 molar ratio of reagents to a produce a 14 wt % solution.Efforts are made to exclude all water—the glassware is first heatedunder a nitrogen flow to remove any adsorbed water. The diamine isdissolved in NMP and once stabilised at room temperature the dianhydrideis added to the solution whilst stirring. Continue to stir for 2 hoursand then store in the fridge at 5° C. or less until use.

2) The polymer precursor is sprayed onto the substrate using a gravityfed air gun at room temperature. With the current data the average massapplied is 0.36 mgcm⁻².

3) CNTs are then incorporated into the polymer precursor while still‘wet’—this is essential for successful CNT incorporation. To do thisCNTs are dispersed in ethyl acetate (at a concentration of 1 mg/ml)using a high shear mixer and sprayed onto the surface of the polymerprecursor at room temperature. It is assumed that the mass of CNTsdeposited onto the surface is 0.1 mg cm⁻².

4) The sample is then ‘dried’ at 60 C for 60 minutes. Although longertimes are typically used this is sufficient for our film thickness.

5) The sample is then heated to 300 C for 20 minutes.

6) CNTs are then exposed using a CF₄/O₂ etch (flow rate ratio of 1:4sccm) for a given time. Typical exposed CNT layer has a thickness of 600nm to 1000 nm.

Explanations:

1) This polymer was chosen for its shock resistance, high thermaldecomposition temperature, its anisotropic mechanical strength and thesimilarity of thermal expansion between the resultant polymer system toaluminium (c.f. 5.0×10{circumflex over ( )}(−5) and 2.4×10{circumflexover ( )}(−5)/K respectively).

This polymer has the same chemical structure as Kapton and is thereforewell known in the literature. Factors within its synthetic process(mainly weight % concentration) were altered so that we could obtain asuitable viscosity for our purpose.

Other polar aprotic solvents could be used as a support medium. DMAc andDMF are commonly used and a THF/MeOH mix has also been shown to act as asolvent system. The reason NMP was selected was that it is the solventmost commonly used for this polymer system. It has a high boiling pointwhich allows enough time for CNT incorporation before evaporation. It isalso believed that as NMP is such a good solvent for CNTs it aids thedispersion of CNTs, once in the polymer matrix, and facilitates theiruniform incorporation throughout the polymer bulk during the drying andcuring stages. This results in a more homogenous composite material.

2) We have shown that an alternative method of applying the polymerprecursor onto the substrate is by spin coating. Other alternativesinclude drop coating, dip coating and to a certain extent painting. Tooptimise conditions for each of these methods the viscosity of thepolymer has been altered. For our change from spin coating to sprayingthis was done by synthesising at different concentrations. This could bedone in two ways: a) Direct synthesis at low wt %—This should achieve areduced viscosity by decreasing the polymer chain length. b) Synthesisat high wt % and then dilution—The polymer chain length here should beretained however the dilution to a final low wt % loading will allow forthe reduced viscosity. In our case we opted for solution a). The reasonthat b) was not chosen is that in order to achieve a reasonableviscosity for spraying, from the current synthetic method, the final wt% loading of the solution would be so low that in order to achieve areasonable final film thickness (around 10% of the applied thicknessafter solvent loss) an unreasonable quantity of solution would have tobe applied. This might come with its own problems such as dripping andthus non-uniformities.

This demonstrates that there is a wide range of possible preparativemethods to achieve the same result.

We believe that the polymer precursor film, prior to drying, is of theorder of 15 μm thick.

3) We have shown that if the polymer precursor is allowed to ‘dry’before the CNTs are applied then they are not successfully incorporatedinto the polymer. However, if applied when ‘wet’ then the CNTs areincorporated throughout the bulk of the resultant polymer film.

The CNT loading achieved via our method reaches up to 20 wt %. This isan average taken from a range of coupons (substrates) assuming that thepolyimide mass is constant at its stated value. This is because theycannot be measured independently with any accuracy due to solvent stillpresent.

To apply the CNTs we currently use the ethyl acetate. NMP is a goodsolvent for CNTs and therefore could also be used. The main reason thatethyl acetate is advantageous is that it does not add to the filmthickness as the solvent evaporates, it does not react with the polymerprecursor (as it is aprotic), as it evaporates it allows you to judgewhen a sufficient amount of CNTs have been applied and it is cheaper. Noprotic solvents can be used as these will react with the poly(amic) acidprecursor thus preventing the curing stage.

4) In the literature ‘drying’ of the polymer precursor solution, priorto curing, is usually carried out in a dry nitrogen oven and over alonger time period. However, we conducted a mass analysis to show thatno additional NMP was lost after just one hour. This, however, isspecific to our system and changing film thickness and polymer wt %would affect this time. Although the majority of NMP is lost in thisstep by evaporation there is a certain quantity that will always remainuntil the curing stage due to coordination to the amic acid.

We have also demonstrated, by way of spherical components, that thedrying and curing process can also be done with a heat gun. As long asthe substrate is heated to the required temperature and does notovershoot then this should be feasible on all substrate shapes.

5) Again, this process has been developed to suit our system. It doesnot work for thicker films, as we have seen cracking during the curingstage (as mentioned above). Degree of curing was measured by IRspectroscopy and it was shown that the same end result, at least byappearance and IR spectroscopy, could be achieved by directly heating to300 C as opposed to a temperature ramp process over 5 hours.

6) The result of the above steps is a grey composite film which can betouched with no damage. The key part of the process to make it black isthe final etch. The etch used is a CF₄/O₂ etch (flow rate ratio of 1:4sccm) which is selective for polyimide. We currently use a power densityof 0.611 W cm-2 for between one and two minutes.

The teachings herein are used to make a variety of different articlesincluding, for example for tracking systems, optical detectors andoptical telescopes, scientific instrumentation and so on.

All optional and preferred features and modifications of the describedembodiments and dependent claims are usable in all aspects of theinvention taught herein. Furthermore, the individual features of thedependent claims, as well as all optional and preferred features andmodifications of the described embodiments are combinable andinterchangeable with one another.

The disclosures in British patent application numbers 1515270.5,1515694.6, 1516423.9 and 1602031.5, from which this application claimspriority, and in the abstract accompanying this application areincorporated herein by reference.

The invention claimed is:
 1. A method of coating a substrate, comprisingthe steps of: (i) providing a suspension of carbon nanostructures in asolvent; (ii) pre-heating a substrate to a sufficient temperature tocause the solvent to evaporate when the suspension contacts thesubstrate; and then (iii) spray-coating the suspension onto thesubstrate; (iv) maintaining the substrate at a sufficient temperatureduring step (iii) in order to maintain evaporation of the solvent beingspray-coated; (v) continuing with steps (iii) and (iv) until a layer ofcarbon nanostructures has been coated on the substrate with a thicknessof at least 2 micrometres to form a non-transparent coating; and (vi)creating an open structure of non-aligned carbon nanotubes and reducingthe bulk density of the coating of step (v) using plasma, therebyproducing a black coating with lower reflectivity than the coating ofstep (v).
 2. A method as claimed in claim 1, wherein the carbonnanostructures are multi-walled carbon nanotubes.
 3. A method as claimedin claim 1, wherein the coating of step (v) has a mass per unit area ofat least 0.07 mgcm⁻².
 4. A method as claimed in claim 1, wherein thetemperature of the substrate in steps (ii) and (iii) is independently atleast 60% of the boiling point of the solvent in ° C. at standardatmospheric pressure.
 5. A method as claimed in claim 1, wherein thetemperature of the substrate in steps (ii) and (iii) is independentlynot more than 4 times the boiling point of the solvent ° C. at standardatmospheric pressure.
 6. A method as claimed in claim 1, wherein thetemperature of the substrate in steps (ii) and (iii) is independentlyfrom 80° C. to 300° C.
 7. A method as claimed in claim 1, wherein theconcentration of carbon nanostructures in the solvent in step (i) arefrom 0.1 to 5 mg/ml.
 8. A method as claimed in claim 1, wherein in step(vi) the plasma etching takes place for a time from 3 to 10 minutes. 9.A method as claimed in claim 1, wherein in step (vi) the plasma etchingtakes place at a pressure from 0.1 to 5 Torr.
 10. A method as claimed inclaim 1, wherein in step (vi) the plasma etching takes place at a powerdensity from 0.02 Wcm² to 4 Wcm².
 11. A method as claimed in claim 1,including the step of providing protective functionalization to thecoating of carbon nanostructures.
 12. A method as claimed in claim 1,including the step of providing a protective overcoat to the coating ofcarbon nanostructures.
 13. A method as claimed in claim 1, including thestep of etching or anodizing the contact surface of the substrate priorto depositing the layer of carbon nanostructures.
 14. A method asclaimed in claim 1, including the step of roughening the contact surfaceof the substrate prior to depositing the layer of carbon nanostructures.15. A method as claimed in claim 14, wherein surface roughening is bywet chemical etching or by impacting the surface with grit.
 16. A methodas claimed in claim 1, including the step of providing a conversioncoating to the roughened surface of the substrate prior to depositingthe layer of carbon nano structures.
 17. A method as claimed in claim 1,including the step of applying a bonding or adhering layer to thecontact surface of the substrate prior to depositing the layer of carbonnano structures.
 18. A method as claimed in claim 17, wherein thebonding or adhering layer is a layer of polyimide, epoxy or polyamidepolymer.
 19. A method as claimed in claim 17, including the step ofheating or curing the bonding or adhering layer during the step ofdepositing the layer of carbon nanostructures thereon.
 20. A method asclaimed in claim 1, wherein the optical spacer of step (vi) istransmissive or absorbing of a frequency of radiation desired to beabsorbed by the coating.
 21. A method as claimed in claim 1, wherein theoptical spacer of step (vi) is or includes zinc sulphide, zinc selenide,styrene, amorphous carbon, naphthalene nanoparticles, or any combinationthereof.
 22. A method as claimed in claim 1, wherein the optical spacerof step (vi) is in the form of particles having an average diameter ofnanometres to tens of micrometres.
 23. A method as claimed in claim 1,wherein the solvent has a polarity index from 0.2 to 6.7.
 24. A methodas claimed in claim 1, wherein the solvent has a polarity index from 3.0to 6.2.
 25. A method as claimed in claim 1, wherein the solvent ischloroform, ethanol, ethyl acetate, acetone, cyclohexane,tetrahydrofuran, dimethyl formamide or N-methyl-2-pyrrolidone.
 26. Amethod as claimed in claim 1, wherein the solvent is free ofsurfactants.
 27. A method as claimed in claim 1, including the step ofetching the dried carbon nanostructure layer.
 28. A method as claimed inclaim 1, including the step of applying a hydrophobic coating to thecarbon nanostructure layer.
 29. A method as claimed in claim 28, whereinthe hydrophobic coating is applied by means of a plasma etching stepfollowing step (vi).
 30. A method as claimed in claim 29, wherein thegeneration of the plasma for said plasma etching step takes place in theabsence of a source of hydrogen atoms, the substrate is heated to atleast 100° C., the power density of the plasma is not more than 0.5 Wcm²and the plasma is generated for a period from 3 to 12 minutes.
 31. Amethod as claimed in claim 1, wherein the suspended carbonnanostructures in said solvent are allowed to agglomerate before beingcoated onto the substrate.
 32. A method as claimed in claim 31,including the step of sonicating the carbon nanostructures in order todisperse them in the solvent and then allowing the suspension to standuntil it agglomerates.
 33. A method as claimed in claim 1, wherein thesubstrate of step (ii) is made by: (i) coating a substrate with apolymer solution which is a polymer dissolved in a first solvent; (ii)providing a suspension of carbon nanostructures in a second solvent;(iii) spray-coating the suspension onto the substrate so that the carbonnanostructures are incorporated into the polymer solution; (iv) dryingthe substrate to evaporate the first and second solvents to leave anon-transparent coating of polymer and carbon nanostructures; (v) plasmaetching the coating to remove at least some of said polymer.